The present invention relates to an ultrasound diagnostic apparatus suited for a contrast echo method using an ultrasound contrast agent mainly composed of microbubbles.
An ultrasound image diagnostic apparatus has many advantages which other modalities, e.g., an X-ray diagnostic apparatus, X-ray computer tomography apparatus (CT scanner), magnetic resonance imaging apparatus (MRI), and nuclear medicine diagnostic apparatus (gamma camera, SPECT, and PET), do not have. For example, the ultrasound image diagnostic apparatus can generate and display images almost in real time concurrently with scanning. This apparatus is compact and inexpensive. In using the apparatus, there is no chance of exposure to radiation. The apparatus can easily visualize blood flow.
(Contrast Agent)
Ultrasound contrast media are now being put into practice, and an improvement in the precision of vascularity examination using such a contrast agent is expected. A contrast agent mainly consists of microbubbles of several microns. Microbubbles rapidly shrink and collapse upon reception of strong ultrasound waves. The contrast enhance effect therefore lasts only for a relatively short period of time.
In actual scan operation, since fresh contrast agent is continuously supplied to a scan slice over a blood flow, a certain degree of contrast enhance effect may be maintained. In general, however, ultrasound waves are applied several thousand times per second. In organs in which the blood flow rate is low or a relatively thin blood vessel, a contrast enhance effect can be maintained only for a moment.
(Intermittent Transmission Method)
That a contrast agent instantaneously collapses upon application of ultrasound waves is the most serious problem in the contrast echo method. As a technique of solving this problem, an intermittent transmission method is available. In this technique, scanning is intermittently repeated in synchronism with the R waves in an electrocardiogram. Fresh contrast agent continuously flows into a slice in the intervals between scans. This ensures a contrast enhance effect for every scan.
(Harmonic Imaging)
The utility of the above contrast echo method is augmented if it is used in combination with a harmonic imaging method. Microbubbles vibrate nonlinearly when ultrasound waves collide with them. This nonlinear vibrations produce harmonic components having frequencies of integer multiples of a fundamental frequency. Harmonic imaging is an imaging technique of extracting harmonic components from a fundament frequency component and visualizing them. Organs hardly vibrate nonlinearly, and hence harmonic components from the organs is pelatively small. Consequently, a region where a contrast agent exists is relatively emphasized.
As described above, if the intermittent transmission method is used, since fresh microbubbles flow into a slice during scan intervals, a contrast enhance effect can be maintained. On the other hand, in order to ensure a certain spatial resolution, about 100 to 200 ultrasound scanning lines are required per frame. That is, ultrasound pulses are applied at least a number of times equal to the number of ultrasound scanning lines in one scan.
In scanning, ultrasound pulses are transmitted while their directions are gradually changed. Therefore, microbubbles on a scanning line adjacent to a scanning line having undergone transmission/reception may not collapse. In practice, however, since an ultrasound beam has a certain width, most microbubbles on the adjacent scanning line collapse in many cases.
In addition, the width of an ultrasound beam changes in accordance with depth. The contrast enhance effect therefore may change depending on depth. Specific problems will be described below with reference to FIGS. 1A and 1B.
When ultrasound pulses are generated from a plurality of arrayed transducers 51 while the generation timing is gradually shifted, a convergent sound field is formed. The depth of ultrasound focus point can be arbitrarily changed by changing the timing. Referring to FIG. 1A, when a focus point is formed on a scanning line with an arrow 52 at a relatively short distance, a region exhibiting a relatively high pressure sound, i.e., a region 53 exhibiting a relatively high degree of microbubble collapse (microbubble collapse region), is indicated by the hatching. The dotted lines represent adjacent ultrasound scanning lines 54.
As shown in FIG. 1A, a short-distance focus point is often formed by driving only transducers near the center instead of driving all the transducers 51. In this case, in the short-distance region, the ultrasound beam is narrow, and hence the microbubble collapse range 53 is also narrow. In the long-distance region, the ultrasound beam is relatively wide, but the ultrasound scanning line pitch is also large. In addition, ultrasound waves greatly attenuate because of the long propagation distance. As a result, the microbubble collapse range 53 becomes relatively narrow.
FIG. 1B shows a microbubble collapse range 55 when a focus point is formed at a relatively long distance. AS shown in FIG. 1B, to ensure a certain degree of sound pressure at the focus point even at a long distance with a long propagation distance, many transducers must be driven at a high voltage. In this case, in the short-distance region, the ultrasound beam becomes wide and the microbubble collapse range 55 also becomes relatively wide. As a consequence, microbubbles on adjacent ultrasound scanning lines collapse. When, therefore, ultrasound transmission/reception is performed afterward in this scanning line direction, microbubbles will have collapsed and been lost.
Another problem will be described next. Consider a case wherein a cardiac minor axis image is obtained by the contrast echo method. As shown in FIG. 2, if a focus point is set at a short distance in accordance with a depth 61 of a cardiac muscle front wall portion 61, a rear wall portion 62 is hardly visualized due to biological damping and a curtain effect due to microbubbles filling a cardiac cavity.
If a focus point is set at a long distance in accordance with the rear wall portion 62, applied ultrasound pulses collapse microbubbles on adjacent ultrasound scanning lines in the short-distance region. Therefore, no contrast enhance effect can be expected at the front wall portion 61.
It is known that in the sector scan method, when ultrasound waves are transmitted in a direction greatly shifted from a direction perpendicular to the probe vibration surface, a decrease in sound pressure and an increase in side lobe level occur as compared with application of ultrasound wave in the direction perpendicular to the problem vibration surface. That is, when ultrasound waves are applied to a cardiac muscle side wall portion 63, side lobes collapse microbubbles near the front wall portion 61 and rear wall portion 62.
It follows from the above description that it is practically impossible to transmit ultrasound waves so as to form a uniform sound field distribution on an entire scan slice. The luminance reference level is therefore not uniform within the scan slice and varies. Since scattering by tissue exhibits a relatively linear response, luminance can theoretically be made uniform by level correction for reception signals. However, a response of microbubbles exhibits strong nonlinearity, and phenomena such as nonlinear vibrations, expansion, and collapse of microbubbles, vary in a complicated manner depending on the absolute level of applied sound pressure. Therefore, it is practically impossible to correct reception signals.